One of the most significant sources of the increase in carbon dioxide concentration in the atmosphere is the burning of fossil fuels in furnaces with the objective of energy conversion. Attempts are therefore made to extract CO2 from the combustion of fossil fuels and subsequently store it, so as to prevent it from entering the atmosphere. The reasons for these efforts are the greenhouse effect and the consequent global warming.
Currently there are three basic concepts for the extraction of carbon dioxide, which differ according to the placement of CO2 extraction in the energy conversion process. They are CO2 extraction after energy conversion (post-combustion), CO2 extraction before energy conversion (pre-combustion) and the generation of a CO2-rich flue gas by way of energy conversion in a flue gas-oxygen atmosphere (the so-called oxyfuel process).
In the oxyfuel process, CO2 is concentrated by condensing the water vapor after combustion of the carbon-containing fuel with pure oxygen. Pure oxygen can be extracted from the air either by way of conventional cryogenic air separation or using a membrane. To limit the combustion temperature, a portion of the flue gas must be reintroduced regularly into the combustion process.
Oxygen-conducting high-temperature membranes have great developmental potential for extracting oxygen from air, in particular due to energy-related considerations. Such an ion-conducting membrane regularly requires an operating temperature ranging between 600 and 1000° C. The driving force for oxygen transport results from the difference in the oxygen partial pressure on the feed side and the permeate side of the membrane unit, and is generally quantified using the Wagner equation. If a sufficient driving force for oxygen transport is present, membrane effects result in concentration of the oxygen on the permeate side.
Membrane-based oxyfuel designs are being developed in numerous research and industrial projects. One of them is the oxycoal-AC design illustrated in FIG. 1, which was developed in a joint project at the RWTH Aachen, so as to develop a CO2 emission-free carbon combustion process for power generation.
A main feature of this design is the combination of two methods for generating driving force in the oxygen-conducting high-temperature membrane unit. Fresh air is compressed on the feed side and subsequently expanded for partial energy recovery, and the permeate side of the membrane is flushed using the recycled flue gas (combustion gas) having a temperature at level of the operating temperature of the membrane unit, thereby removing the permeating oxygen on the permeate side. The oxygen-enriched flue gas which is subsequently fed to the burner is composed mainly of CO2, H2O and O2, and typically has an oxygen content of approximately 17% by volume. Both methods for generating driving force result in a partial pressure differential for oxygen forming between the feed side and the permeate side.
The concept described, however, has a few disadvantages:                Driving the turbo components for air compression results in high energy consumption.        Scrubbing the flue gas to be recycled at temperatures corresponding to the membrane operating temperature (hot gas scrubbing) is currently unavailable on the required scale.        Flue gas blowers for conveying the flue gas to be recycled at temperatures corresponding to the membrane operating temperature (hot gas scrubbing) are currently unavailable on the required scale.        A conventional fresh air-flue gas heat exchanger which operates at a pressure ratio of up to 20 bar to 1 bar can be attained only at high cost.        The high pressure differential between the feed side and the permeate side of the membrane necessitates a high stability membrane.        
A further membrane-based oxyfuel design was developed by Siemens AG, which is referred to in the following as the clean concept and is depicted in FIG. 2. According to the clean concept, the permeate side of the membrane is no longer flushed using the flue gas, in contrast to the oxycoal-AC concept. The oxygen permeating through the membrane is conveyed out of the membrane unit optionally using a pump, and is subsequently added to the flue gas which is returned to the combustion process. Maintenance of the required process temperature of the membrane unit is ensured solely by heating the air after air compression using heat exchange. To ensure the necessary operating temperature of the membrane, the heat exchanger extracts the heat required therefor from the flue gas produced during operation of the burner.
Advantages of this concept from Siemens are the avoidance of hot gas scrubbing, prevention of possible damage to the membrane by the components of the flue gas, shortening of the flue gas recirculation line, and the possibility of integrating the membrane module in first-generation oxyfuel methods which are characterized by oxygen being provided using an air separation system and by cold flue gas recirculation.
This concept also has a few disadvantages, however:                Driving the turbo components for air compression results in high energy consumption.        The process control results in reduction of the driving force through the membrane (with constant energy consumption as in the oxycoal-AC concept).        The high pressure differential between the feed side and the permeate side of the membrane results in high stability requirements on the membrane.        